The story centers on an aging assassin who finds himself in a battle with the ultimate opponent: his clone, who is 25 years younger and at the peak of his abilities.

The project was first set up at Disney in 1997 as a pitch by Darren Lemke, with Don Murphy producing and Tony Scott directing. While always popular with execs, in and outside the studio, Gemini was considered unfilmable for many years because visual effects technology had not caught up to the concept. (Originally the idea was to have an actor such as Harrison Ford play the older assassin and someone like Chris O'Donnell play the younger version, before settling on the idea of one actor playing both parts.) The script has been retooled many times over the ensuing two decades, and numerous camera tests have been conducted by the studio, which at different times has considered Mel Gibson and Clint Eastwood for the lead role.

Skydance picked up the project in October 2016, with Bruckheimer on board and Murphy exec producing, in the belief that the technology now exists to have someone like Smith realistically play two different ages and spar with himself.

Smith's deal is not done, with things like scheduling (the actor is in early talks to play the Genie in Disney's live-action Aladdin project) among the topics that still need to be hased out.